It is known that the performance of devices incorporating semiconductor and insulating photoconductor materials is dependent upon the properties of both the majority and minority charge carriers. While measurement of the properties of the majority carriers is relatively easy, the characteristics of devices using these materials is more often dependent upon the minority carriers. Therefore, measurement of the properties of the minority carriers is important in choosing these materials for manufacture of commercial products such as solar cells, bipolar transistors, photocopiers, etc.
The best known method of determining the properties of minority carriers is to measure the diffusion length of the carriers. This is because both majority and minority carriers must diffuse together in the material to maintain space charge neutrality. Therefore, the minority carrier, which is the slower moving of the two carrier types determines the diffusion rate. A key property of the minority carrier can be obtained by measuring its diffusion length, which is the product of its mobility and lifetime.
In photovoltaic devices such as solar cells, the minority carrier diffusion length is the single most important parameter in determining the performance of the solar cell. A high diffusion length insures a high efficiency in solar cell performance and good quality in the performance of bipolar transistor devices.
Known methods for measuring the diffusion length of the minority carriers in a sample material include the surface photovoltage method (SPV) as described by A. R. Moore in the text Semiconductors and Semimetals, edited by J. I. Pankove, Academic Press, 1984, Vol. 21C, Chap. 7. This method uses optical apparatus capable of emitting light at different wavelengths such as a broad band light source together with a monochromator. In addition, another light source is needed to provide a strong background illumination for the sample material. The material is connected between two contacts, each of which has strict requirements, one of these being a rectifying contact which is transparent to the illumination and the other being ohmic. A voltage produced by the illumination is measured between these contacts and the intensity of the variable wavelength light source is varied so as to keep that voltage constant. The diffusion length is obtained from a plot of the data relating the light intensity and wavelength for constant voltage.
In addition to the complexity of the optical apparatus and the special contacts required for the voltage measurement, the intense background illumination required in this method causes the sample material to degrade during the measuring process which affects the accuracy of the measurement.
Another known method for diffusion length measurement is the transient grating spectroscopy method as discussed by Eichler in Festkorperproblems, Advances in Solid State Physics, Vol. XVIII, p. 241 (1978). In this technique, pulses of two coherent light beams which illuminate the sample material momentarily form an interference pattern which creates a varying concentration of majority and minority carriers and so changes the refractive index of the material. A third probe beam is directed on the sample and is diffracted by this optical grating. The diffracted light is measured as a function of time, and because of the time dependency of the diffracted light and the difficulty in locating the precise spot where the diffraction takes place, neither the method itself nor the analysis of the results is trivial. Furthermore, the results relate to a transient rather than a steady state condition of the sample material, the latter being conditions which are normally encountered in the use of solar cells and similar devices.
It would therefore be desirable to provide an optical apparatus incorporating a simple method of measuring the diffusion length as a key property of insulating photoconductor and semiconductor materials.